1. Field of Invention
The present invention relates to an illumination device and display device using this wherein a light-guide plate is arranged at the front face of an illuminated object and this illuminated object is two-dimensionally surface-illuminated, and more particularly relates to an illumination device and display device using this wherein properties such as illumination function, recognisability, contrast and energy-saving that are based on the optical diffusion characteristic of this light-guide plate are greatly improved.
2. Description of Related Art
Conventionally, various types of illumination device are employed that exhibit the function of surface illumination in respect of a display device that requires planar illumination, such as a liquid crystal display device.
For example there is known a display device that is arranged at the back face of the object to be illuminated, such as a liquid crystal display panel; normally such an illumination device is arranged to be constantly lit. An illumination device is also known mounted on a liquid crystal display device having a reflective function. In the case of such an illumination device, a reflective plate is arranged at the back face of the liquid crystal display panel and the device is employed by illumination provided by external light. Furthermore, an illumination device is also known that is arranged together with a semi-transparent reflective plate at the back face of a liquid crystal display panel; this illumination device is used for reflection when the environment is brighter and to provide back lighting illumination when the environment is dark (for such devices, for example Early Japanese Patent Publication No. SHO.57-049271, Early Japanese Patent Publication No. SH0.57-054926 and Early Japanese Patent Publication No. SH0.58-095780 may be referred to).
However, conventional illumination devices having solely an illumination function suffered from the problem that power consumption in order to keep the light source constantly lit was large; for example, they could not be used over a long time to provide illumination for portable equipment. Also, in the case where a conventional display device having solely a reflective function was mounted on a liquid crystal display device or the like, there was the problem that contrast of the display screen was low, making it impossible to employ them in a dark environment. Furthermore, illumination devices that are employed with a semi-transparent reflective plate inevitably have the problem that the display is dark both when used with reflection and when used with back lit illumination; this technique represents an unsatisfactory compromise and has not become popular at all.
In these circumstances there has recently been proposed for example in Early Japanese Patent Publication No. H.6-324331 an illumination device that is arranged at the front face of a display device such as a liquid crystal display device. The illumination device of this proposal is incorporated in a thin liquid crystal display device and has the object of ensuring high contrast of illumination both when lit and when not lit. Specifically, a thin illumination device is arranged at the top face (front face) of a liquid crystal display and a reflective plate is arranged at the back face of the liquid crystal display. The illumination device comprises a light-guide plate and a light source that is arranged at the end face of this light-guide plate or in its vicinity. At the optical output face of the light-guide plate, there is formed an indented shape comprising faces practically parallel with this face and faces approximately perpendicular thereto. The indented shape may be formed for example of a plurality of ribs or projections of cylindrical or prismatic shape.
However, the illumination devices arranged at the front face of these publications are adapted to light sources of rod or linear shape. For such light sources, fluorescent tubes, which are of high light-emitting efficiency are generally employed; however, fluorescent tubes need power of at least a certain level and suffer from the problem that their power consumption cannot be reduced below this. Also, if point light sources such as LEDs or electric light bulbs were employed, there was the problem that, since the lines of intersection at the root sections forming the ribs or prismatic projections and optical output faces are straight lines, the quality of illumination tends to be adversely affected by regular reflection. Furthermore, in the case of point light sources, there was the problem that unevenness of brightness could not be eliminated by one-dimensional distribution control of the pattern of the projections. Also, illumination devices arranged at the front face in this way were subject to the problem of being easily affected by external damage to the light-guide plate, causing light to be emitted by dispersive reflection of optical flux from such damaged portions, lowering the contrast of the illuminated object such as the liquid crystal display when lit.
Also, with such illumination devices of the type that were arranged at the front face, since the light source is arranged at the end face of the light-guide plate, a space needs to be provided at the end of the light-guide plate sufficient to screen the light source from the observer; if they are employed as illumination for a liquid crystal display or the like, a border is therefore necessary around the periphery of the display area. This resulted in waste of space and imposed considerable design limitations.
In one aspect, the present invention was made in order to solve the various problems of a conventional illumination device as described above.
An object of the present invention is to provide an illumination device using a point light source of low power consumption and high quality, and a display device such as a liquid crystal display device using this.
Also, a further object of the present invention is to provide an illumination device of low power consumption and high quality by employing as light source a light emitting diode (LED), and a display device such as a liquid crystal display device using this. Yet a further object of the present invention is to provide an illumination device whereby illumination can be achieved without loss of reflective function and a display device such as a notice board device or liquid crystal display device using this, and a device such as an electronic device or mobile telephone using this liquid crystal display device.
Yet a further object of the present invention is to provide an illumination device with little deterioration of illumination function by low-cost, convenient means, and a display device such as a liquid crystal display device with little deterioration of display quality.
Yet a further object of the present invention is to provide an illumination device whereby rays of light can be efficiently directed into the interior of a light-guide plate from a light source positioned remote from the light-guide plate end, which is space-saving, and has excellent design characteristics, a display device such as a liquid crystal display device, and a device such as an electronic device or mobile telephone using this liquid crystal display device.
Furthermore, from the point of view of display devices in which an illumination device is mounted, in view of conventional reflective type liquid crystal display devices for the aforesaid display devices, it is an object to provide various types of electronic device such as liquid crystal display devices, portable telephone devices; timepieces, cameras or data terminal devices wherein the production of a bright line which is annoying to the observer can be prevented, wherein unevenness of brightness can be eliminated, which are of a type in which power consumption can be reduced and furthermore which afford an illumination function of high quality.
Furthermore, conventionally, notice board devices having an illumination function were of a construction in which a casing was provided whose front face was covered by transparent glass and wherein a notice was illuminated by arranging a light source at the front edge of the notice. Also, they were of a construction in which the person viewing them could not directly see the light source due to an optical screening section, also serving as a casing, in front of the light source.
However, conventional notice board devices suffered from the problem that they had to be of sufficient thickness in order for the entire notice to be illuminated, and that the difference of illumination was large at locations remote from the vicinity of the light source.
Also, in a further aspect relating to a display device, an object of the present invention is to provide a notice board device wherein such problems are solved and which is of small thickness and wherein the uniformity of illuminance is high.
In order to solve the aforementioned problems, in one aspect thereof, the present invention provides an illumination device arranged at the front face of an illuminated object, comprising a light-guide plate of transparent flat plate form formed with point-form optical extraction structures on its surface on at a position facing this surface, and a light source arranged facing the end face of this light-guide plate. Suitably the light source is a point light source. Also for example the optical extraction structures are distributed relatively sparsely in the vicinity of the point light source and progressively more densely going away from the point light source. Further, a rod-shaped optical diffuser may be provided arranged between the end face of the light-guide plate and the point light source. For example a milky white transparent body in which optical diffusing material is dispersed may be employed as the rod-shaped diffuser. Also for example a transparent body formed with optical extraction shapes may be employed as the rod-shaped optical diffuser. As an example, rib-shaped projections may be provided as the optical extraction structures on the optical output face facing the illuminated object of the light-guide plate. Further, pillar-shaped projections may be provided as the optical extraction structures on the optical output face facing the illuminated object of the light-guide plate. Also, as a further aspect, in a liquid crystal display device comprising a liquid crystal display and an illumination device arranged at the front face of this liquid crystal display, the illumination device comprises a light-guide plate forming a transparent flat plate shape and formed with point-form optical extraction structures on its surface or at a position facing this surface, and a point light source arranged facing the end face of this light-guide plate.
Further, as another aspect, there is provided a display device arranged at the front face of an illuminated object comprising a light-guide plate formed of transparent plate shape and formed at its surface with projections or concavities for optical diffusion having an inclined surface of under about 30xc2x0 with respect to this surface, and a light source arranged opposite the end face of this light-guide plate. In this case, a reflecting member can be arranged adjacent the other end face of the light-guide plate apart from the end face where the light source is arranged. Also, a reflecting member may be arranged so as to cover this light source and the end face of the light-guide plate where the light source is arranged.
Also, as a further aspect of an illumination device according to the present invention, in an illumination device wherein optical flux from a light source is transmitted in one direction practically orthogonal to the plate face direction and that is provided with a light-guide plate that delivers this optical flux as illumination from a face in this one direction, a sheet-form transparent member is arranged facing the face of the light-guide plate on the opposite side to this one direction.
As another aspect there is further provided; in an illumination device arranged at the front face of an illuminated object, a light-guide plate formed with optical extraction structures at its surface and constituting a transparent sheet-shaped member, and a light emitting diode (LED) arranged integrally with an end face of this light-guide plate. Suitably, the optical extraction structures are distributed relatively sparsely in the vicinity of the light emitting diode and progressively more densely going away from the light emitting diode. Also, pillar-shaped projections could be provided as the optical extraction structures on a face facing the illuminated object of the light-guide plate. Concave shapes or convex-shaped projections could be provided as the optical extraction structures on a face opposite to the face facing the illuminated object of the light-guide plate. Also as a further aspect, in an illumination device arranged at the front face of an illuminated object, there are provided a light-guide plate formed with optical extraction structures at at least one face of the transparent flat plate, a light source arranged at a position remote from the same planar position as this light-guide plate, and means for converting that convert the direction of the rays from this light source so that they are guided into the optical input face of the light-guide plate.
Yet further, in an aspect of the present invention relating to a display device, as one mode thereof, in a liquid crystal display device of the reflective type wherein at least a front light is arranged at the top face and a polarisation separating plate is arranged at the bottom face, this front light comprises a light-guide plate comprising a flat transparent plate formed with a plurality of point-form optical extraction elements in its surface, and a point light source arranged facing the end face of this light-guide plate. For example these optical extraction structures may be distributed relatively sparsely in the vicinity of the light source and progressively more densely going away from the light source. Also suitably a rod-shaped optical diffuser is provided arranged at the end face of the light-guide plate and the point light source is arranged at the end of this rod-shaped optical diffuser. Also suitably pillar-shaped projections are provided as the optical extraction structures on a face facing the illuminated object of the light-guide plate. Furthermore, concave shapes or convex shapes could be provided as optical extraction structures on the face on the opposite side to the face facing the illuminated object of the light-guide plate.
In yet a further aspect of a display device according to the present invention, in a notice board device having an illumination function, a transparent plate is arranged at the front face of the notice and a function of planar illumination of this notice is conferred on this transparent plate.